C++ Runtime _best_ May 2026
: The stack is unwound again. For each frame between the throw site and the catch frame, the personality routine calls destructors for local objects ( _UA_CLEANUP_PHASE ). Finally, control jumps to the catch block.
(simplified for a class hierarchy A <- B ): c++ runtime
vtable for B: [0] offset_to_top = 0 [1] typeinfo pointer -> typeinfo for B [2] A::foo() thunk (if overriding) vtable for A: [0] offset_to_top = 0 [1] typeinfo pointer -> typeinfo for A : The stack is unwound again
Report ID: CPP-RT-2024-01 Date: April 14, 2026 Author: Systems Software Research Division Subject: Structure, Execution Flow, and Overhead of the C++ Runtime Environment 1. Executive Summary The C++ runtime is the set of software components that support the execution of a compiled C++ program beyond the raw machine code generated by the compiler. Unlike C, which has a relatively minimal runtime, C++ requires substantial behind‑the‑scenes machinery to implement core language features: dynamic initialization of globals, exception handling, run‑time type information (RTTI), new / delete operators, and stack unwinding. This report dissects the C++ runtime into its constituent parts, traces the execution flow from _start to main and beyond, analyzes the cost of each runtime feature, and examines implementation variations across major compilers (GCC/Clang, MSVC) and operating systems. 2. Components of the C++ Runtime The C++ runtime is not monolithic. It consists of several logical layers: (simplified for a class hierarchy A <- B
main → foo() → bar() → throw MyException(); → __cxa_throw (sets unwind info, calls _Unwind_RaiseException) → _Unwind_RaiseException → _Unwind_RaiseException_Phase1 (walks .eh_frame) → _Unwind_RaiseException_Phase2 (calls personality, destructors) → __gxx_personality_v0 (calls destructors of locals in bar, foo) → land on catch in main On Linux: readelf -S a.out (look for .init_array , .eh_frame , .gcc_except_table ) On Windows: dumpbin /HEADERS myapp.exe (look for .pdata for x64 exception tables) This report is based on the Itanium C++ ABI (version 1.2), Microsoft C++ ABI (x64), and publicly available documentation for GCC 13, Clang 17, and MSVC 2022.
// Constructor function for global std::string _GLOBAL__sub_I_example.cpp: stp x29, x30, [sp, -32]! mov x0, :got:globalString bl _ZNSt7__cxx1112basic_string...C1E... // std::string::string(char const*) ldr x0, :got:__dso_handle adrp x1, :got:_ZNSt7__cxx1112basic_string...D1E... add x1, x1, :lo12:_ZNSt7__cxx1112...D1E... bl __cxa_atexit // register destructor for atexit ldp x29, x30, [sp], 32 ret // .init_array entry points to this function
